Fix race in filter_search().

Using the critical section seems the easiest fix for this. Add a unit test that fails TSAN check if the fix is not applied.
python · nascheme · Apr 9, 2025 · Dec 28, 2024 · Dec 21, 2024 · Feb 7, 2025
commit f79daaa392c854880ce8af16ee0d0f17503f3a56
diff --git a/Lib/test/test_free_threading/test_races.py b/Lib/test/test_free_threading/test_races.py
@@ -5,6 +5,7 @@
 import time
 import unittest
 import _testinternalcapi
+import warnings
 
 from test.support import threading_helper
 
@@ -286,5 +287,36 @@ def set_recursion_limit():
         do_race(something_recursive, set_recursion_limit)
 
 
+@threading_helper.requires_working_threading()
+class TestWarningsRaces(TestBase):
+    def setUp(self):
+        self.saved_filters = warnings.filters[:]
+        # Add multiple filters to the list to increase odds of race.
+        for lineno in range(20):
+            warnings.filterwarnings('ignore', message='not matched', category=Warning, lineno=lineno)
+        # Override showwarning() so that we don't actually show warnings.
+        def showwarning(*args):
+            pass
+        warnings.showwarning = showwarning
+
+    def tearDown(self):
+        warnings.filters[:] = self.saved_filters
+        warnings.showwarning = warnings._showwarning_orig
+
+    def test_racing_warnings_filter(self):
+        # Modifying the warnings.filters list while another thread is using
+        # warn() should not crash or race.
+        def modify_filters():
+            time.sleep(0)
+            warnings.filters[:] = [('ignore', None, UserWarning, None, 0)]
+            time.sleep(0)
+            warnings.filters[:] = self.saved_filters
+
+        def emit_warning():
+            warnings.warn('dummy message', category=UserWarning)
+
+        do_race(modify_filters, emit_warning)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Python/_warnings.c b/Python/_warnings.c
@@ -424,7 +424,10 @@ filter_search(PyInterpreterState *interp, PyObject *category,
               PyObject *text, Py_ssize_t lineno,
               PyObject *module, char *list_name, PyObject *filters,
               PyObject **item, PyObject **matched_action) {
-    /* filters list could change while we are iterating over it. */
+    bool result = true;
+    *matched_action = NULL;
+    /* Avoid the filters list changing while we iterate over it. */
+    Py_BEGIN_CRITICAL_SECTION(filters);
     for (Py_ssize_t i = 0; i < PyList_GET_SIZE(filters); i++) {
         PyObject *tmp_item, *action, *msg, *cat, *mod, *ln_obj;
         Py_ssize_t ln;
@@ -434,7 +437,8 @@ filter_search(PyInterpreterState *interp, PyObject *category,
         if (!PyTuple_Check(tmp_item) || PyTuple_GET_SIZE(tmp_item) != 5) {
             PyErr_Format(PyExc_ValueError,
                          "warnings.%s item %zd isn't a 5-tuple", list_name, i);
-            return false;
+            result = false;
+            break;
         }
 
         /* Python code: action, msg, cat, mod, ln = item */
@@ -450,43 +454,49 @@ filter_search(PyInterpreterState *interp, PyObject *category,
                          "action must be a string, not '%.200s'",
                          Py_TYPE(action)->tp_name);
             Py_DECREF(tmp_item);
-            return false;
+            result = false;
+            break;
         }
 
         good_msg = check_matched(interp, msg, text);
         if (good_msg == -1) {
             Py_DECREF(tmp_item);
-            return false;
+            result = false;
+            break;
         }
 
         good_mod = check_matched(interp, mod, module);
         if (good_mod == -1) {
             Py_DECREF(tmp_item);
-            return false;
+            result = false;
+            break;
         }
 
         is_subclass = PyObject_IsSubclass(category, cat);
         if (is_subclass == -1) {
             Py_DECREF(tmp_item);
-            return false;
+            result = false;
+            break;
         }
 
         ln = PyLong_AsSsize_t(ln_obj);
         if (ln == -1 && PyErr_Occurred()) {
             Py_DECREF(tmp_item);
-            return false;
+            result = false;
+            break;
         }
 
         if (good_msg && is_subclass && good_mod && (ln == 0 || lineno == ln)) {
             *item = tmp_item;
             *matched_action = action;
-            return true;
+            result = true;
+            break;
         }
 
         Py_DECREF(tmp_item);
     }
-    *matched_action = NULL;
-    return true;
+    Py_END_CRITICAL_SECTION();
+    return result;
 }
 
 /* The item is a new reference. */